THIS PROJECT SERVES AS A MODEL FOR A TYPE OF BIOLOGICAL PROBLEM THAT SEEKS ANSWERS ON A GENOME-WIDE SCALE - THE QUESTION OF WHAT ARE THE DOWNSTREAM SUBSTRATES OF BRANCHING PATHWAYS. ALTHOUGH THERE MAY BE HUNDREDS OF PROTEINS WHOSE DEGRADATION IS REGULATED BY THE SO-CALLED N-END RULE, ONLY A SINGLE YEAST PROTEIN THAT IS THE SUBSTRATE FOR THIS MARVELOUSLY COMPLEX PATHWAY HAS BEEN IDENTIFIED BY TRADITIONAL BIOLOGICAL METHODS. THE ENTIRE BASIS FOR THE N-END RULE HAS BEEN THE BEHAVIOR OF HETEROLOGOUS MARKER PROTEINS INTRODUCED INTO YEAST; THESE STUDIES HAVE ELABORATED THE COMPELX PATHWAY OF N-END SIGNALED UBITUITIN-MEDIATED PROTEIN DEGRADATION. BUT WHAT IS MISSING FROM THE STORY IS THE SUBSTRATES THAT THIS UNUSUAL PATHWAY OPERATES ON, SO IT IS HARD TO KNOW WHAT THE FUNCTION(S) OF THE PATHWAY ARE.